l. Field of the Invention
The present invention relates to a compact zoom lens with high zoom ratio suitable for lens shutter cameras or video cameras.
2. Related Background Art
There are a variety of zoom types recently proposed for the purpose of size reduction and increase of zoom ratio in zoom lenses used for lens shutter cameras or video cameras. In the zoom lenses, a negative lens group is arranged closest to an image plane for reduction of total length and compensation of curvature of field.
In the zoom lenses there is known a zoom type composed of two lens groups having a positive refracting power and a negative refracting power, respectively. Japanese Laid-open Patent Application No. 62-235916 discloses such a zoom type, in which the positive lens group includes three separate lens groups, i.e., the la lens group of negative refracting power, the 1b lens group of positive refracting power and the 1c lens group of positive refracting power and in which in zooming a gap decreases between the 1a lens group and the 1b lens group while a gap increases between the 1b lens group and the 1c lens group.
Also, Japanese Laid-open Patent Application No. 63-271214 discloses another zoom type, in which there are three lens groups, i.e., the first lens group of negative refracting power, the second lens group of positive refracting power and the third lens group of negative refracting power arranged in the named order from the object side and in which in zooming from the wideangle end to the telephoto end a gap decreases between the first lens group and the second lens group and a gap also decreases between the second lens group and the third lens group, whereby the changes of image magnification of the first lens group and the third lens group are always used for increasing the total magnification.
Further, Japanese Laid-open Patent Application No. 3-240015 discloses another zoom type composed of four lens groups which are negative, positive, positive and negative in the named order from the object side, thereby to achieve a high zoom ratio.
In the arrangement as disclosed in Japanese Laid-open Patent Application No. 62-235916, the image magnification of each lens group changes in zooming from the wideangle end to the telephoto end such that that of the second lens group decreases but that of the third lens group increases. Since there is almost no change in overall refracting power of the first to third lens groups, the substantial zooming is carried out only by the fourth lens group. Accordingly, to increase the zoom ratio in this arrangement, the image magnification of the fourth lens group must have a great change, which is not preferable in that aberration change is unavoidable in zooming.
In the arrangement as disclosed in Japanese Laid-open Patent Application No. 63-271214, the image magnification is changed in zooming by the second lens group and the third lens group, which is favorable for increase of zoom ratio in respect of arrangement of refracting power. However, since the refracting power of the second lens group is very strong in positive, angles of off-axial rays passing through the second lens group change greatly in zooming, but the heights of off-axial rays scarcely change. Therefore, the change of off-axial aberration is unavoidable in zooming, which makes the increase of zoom ratio difficult.
In the arrangement as disclosed in Japanese Laid-open Patent Application No. 3-240015, the composite refracting power of the third lens group and the fourth lens group is a little positive at the wideangle end, which results in failing to keep back focus sufficient. Also, since the gap between the first lens group and the second lens group changes greatly in zooming, the total length and the effective diameter of the lens closest to an object (as will be referred to as front lens effective diameter) become huge at the wideangle end, which is unsatisfactory in respect of size reduction.